Färgrobot
A robot can move along a row of colored squares. Every square can be either red, green or blue. A command to the robot is a color. The robot responds by moving to the right until it stands on the color specified by the command. Write a program that, for a given sequence of squares and a number $N$, writes out the sequence of $N$ commands that makes the robot go as far to the right as possible.
Input
The first line containes an integer $N$, the number of commands to give to the robot. The second line contains a string of letters, R , G, or B, which specifies the color of each square in the row, from left to right. The string contains less than 200 characters, and is always sufficiently long that no sequence of $N$ commands may cause the robot to reach outside the row of squares.
Output
The program should output a string of $N$ characters, each being R, G, or B, which constitutes the sequence of commands you should give to the robot to make it move as far as possible along the row of squares. If there are several sequences that accomplish this, you may output anyone of them.
Limits

In one test case worth 20 points, $N=4$ and the sequence is ’RGBGGBGRBRBRGRGRB’ (i.e. the case shown on this year’s PO poster)

In other test cases worth 30 points, $1 \le N \le 4$.

In test cases worth 20 points, $5 \le N \le 10$.

In test cases worth 30 points, $20 \le N \le 25$.
Sample Input 1  Sample Output 1 

1 RRBBBGG 
G 
Sample Input 2  Sample Output 2 

2 GBBRGRGBRG 
RB 
Sample Input 3  Sample Output 3 

3 RGBGRRBGBBRG 
BBR 
Sample Input 4  Sample Output 4 

4 RBRBRBBRBRBRGRGGRGGGRBBRBRRBBRBRBRGBGBGBBGBGBGBBBGBGR 
GBGR 